In the prior art, Plow, U.S. Pat. No. 4,408,273, "Method and Means for Cataloging Data Sets Using Dual Keyed Data Sets and Direct Pointers", issued Oct. 4, 1983, describes various index sequential access methods, while Wiederhold, "Database Design", McGraw-Hill Publishing Co., pp. 182-189, copyright 1977, teaches some VSAM fundamentals. The data structures described by Wiederhold include the base clusters which are organized (cataloged) in the data structures used in the method described by Plow. Reference should also be made to IBM MVS/XA VSAM Users Guide, GC26-44015, page 39, published 1983, and IBM Integrated Catalog Facility, GC26-4019, pp. 178-188, published 1983. These IBM publications disclose VSAM data set transfer commands at the logic level. They also disclose the external interfaces for the access method services (AMS) utility.
The prior art, as exemplified by Gotlieb and Gotlieb, "Data Types and Structures", Prentice Hall, Inc., copyright 1978, describe film indices and directories at pages 330-339.
In discussing the fact that VSAM is a two-level or nested access method formed from an index-sequential access method and files (sequence set) nested within a B-tree (index set), Gotlieb et al point out at pages 338-339:
"Suppose that a B tree is grown as follows:
1. Only records (and their keys) are inserted. PA1 2. A record can never leave the level (lowest) at which it was entered; when a (terminal) node of records splits, only the middle key value is passed up to the next level; the associated record (plus key) stays in one of the halves (say the left). PA1 3. Each terminal node is linked to its lexical successor . . .
The result is an indexed-sequential file which grows by cellular splitting. Point 3 enables the file to be processed sequentially, independent of the index (except to find the starting record). This is essentially the organization used by IMB's VSAM (Virtual Storage Access Method) to store records in key order . . . "
In discussing VSAM as an access method utilizing B-tree indices defined onto contiguous portions of DASD storage and capable of accessing partial DASD tracks, Gotlieb et al state at page 339:
"When a B tree is implemented, questions of node size, placement, and creation (due to splitting) must be faced. VSAM, although hardware independent, represents specific design choices made in these regards. Each control interval is a contiguous portion of auxiliary storage; it might be a track, portion of a track, or several consecutive tracks, but it would not, for example, consist of tracks from different cylinders . . . VSAM determines some optimal number of control intervals to be grouped into a control area."
The above-mentioned Plow reference is directed to a method for managing access to a DASD storage subsystem utilizing a VSAM shared among lossely-coupled processors. Plow points out that access to records via a VSAM requires undertaking a two-level address mapping. First, access has to be made to a catalog, and then to an index. Plow characterizes his object as that of maintaining a catalog structure:
". . . residing on the shared data volume, which enables the opening of a user data set on a second data volume even where an external event has changed a data volume, resulting in loss of synchronization between said base catalog and said user data set . . . " (col. 48, lines 2-7)